Along with recent development in miniaturization and speeding-up of electronic equipment, it is becoming common to mount components on a circuit board with a high density. Such high density mounting can be achieved by miniaturization of components to be mounted. In recent years, passive components such as a resistance element and a capacitor mounted in a circuit board, for example, have been miniaturized by employing a technique for embedding the passive components between layers of the circuit board, instead of a conventional technique of mounting the passive components on electrodes on a circuit board surface by using solder.
As methods of manufacturing a resistance element serving as the passive component, a thick-film method and a thin-film method are known.
In the thick-film method, resistance elements are formed between layers of a circuit board by printing a resistor paste between the layers and thereafter baking the paste. Here, the resistor paste is made of epoxy resin and carbon filler particles dispersed in the epoxy resin. However, the thick-film method has a problem that the resistance elements vary in resistance value since the thickness of the paste is likely to vary from one resistance element to another in printing. As countermeasures against this problem, one possibility is to employ a method of adjusting outer shapes of the resistance elements by causing the baked paste to partially evaporate with irradiation of laser beams. However, employing this method causes enormous time and effort in adjusting the shapes of all of a large number of resistance elements to be formed on the circuit board.
In the thin-film method, by contrast, a resistor layer is patterned by wet etching, and thereby resistance elements each having a predetermined shape are formed. The resistance elements thus formed have the same thickness as that of the resistor layer. Accordingly, the problem that the resistance elements vary in thickness does not arise in the thin-film method. However, employing wet etching, which is isotropic etching, causes a problem that controlling outer shapes of the resistance elements after etching is difficult.
Some conventional techniques are disclosed in the specification of Japanese Patent No. 4057589, Japanese Laid-open Patent Publication No. hei 05-205904, and Japanese Laid-open Patent Publication No. hei 06-16441.